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Pesquisa Agropecuária Brasileira

Print version ISSN 0100-204XOn-line version ISSN 1678-3921

Pesq. agropec. bras. vol.38 no.12 Brasília Dec. 2003

http://dx.doi.org/10.1590/S0100-204X2003001200014 

NOTAS CIENTÍFICAS

 

Characterization of a Peruvian isolate of Metarhizium anisopliae var. acridum, a pathogen of grasshoppers

 

Caracterização de um isolado peruano de Metarhizium anisopliae var. acridum, um patógeno de gafanhotos

 

 

Bonifácio Peixoto MagalhãesI; Myrian Silvana TiganoI; Irene MartinsI; Heloisa FrazãoI; Hilda Gómez RamirezII

IEmbrapa-Centro Nacional de Pesquisa de Recursos Genéticos e Biotecnologia, Caixa Postal 02372, CEP 70849-970 Brasília, DF. E-mail: boni@cenargen.embrapa.br, myrian@cenargen.embrapa.br, irene@cenargen.embrapa.br, heloisa@cenargen.embrapa.br
IIServicio Nacional de Sanidad Agraria, Pasaje Francisco de Zela, s/n, Piso 10, Edif. Min. de Trabajo, Lima 11, Peru. E-mail: hgomez@senasa.gob.pe

 

 


ABSTRACT

The objective of this study was to characterize the Peruvian isolate of Metarhizium anisopliae var. acridum, CG 863, obtained from the grasshopper Schistocerca interrita, a crop pest in Peru. The characterization was done by comparing this isolate with two other ones of M. anisopliae var. acridum, from Brazil and Australia, and with an isolate of M. anisopliae var. anisopliae. The three M. anisopliae var. acridum isolates had similar growth profiles in agar plates at 25°C and 37°C, and similar RAPD patterns according to the analysis of three primers. However, regarding these parameters and conidial size, these isolates were very distinct when compared to M. anisopliae var. anisopliae isolate. Bioassays indicated that the Peruvian isolate is as pathogenic as the Brazilian isolate against nymphs of Rhammatocerus schistocercoides.

Index terms: Acrididae, insecta, entomopathogenous fungi, pest control.


RESUMO

O objetivo deste trabalho foi caracterizar o isolado peruano de Metarhizium anisopliae var. acridum, CG 863, obtido do gafanhoto Schistocerca interrita, praga prejudicial a muitas culturas no Peru. A caracterização foi realizada pela comparação com dois outros isolados de M. anisopliae var. acridum, do Brasil e da Austrália, e com um isolado de M. anisopliae var. anisopliae. Os três isolados de M. anisopliae var. acridum mostraram padrões de crescimento semelhantes em placas de ágar a 25°C e 37°C, e semelhantes padrões de RAPD obtidos com a análise de três primers. Entretanto, com relação a estes parâmetros e tamanho de conídios, esses isolados diferiram do isolado de M. anisopliae var. anisopliae. Bioensaios indicaram que o isolado peruano é tão patogênico a Rhammatocerus schistocercoides quanto o isolado brasileiro.

Termos para indexação: Acrididae, inseto, fungo entomopatogênico, combate às pragas.


 

 

Grasshopper outbreaks have occurred in South America for many decades (Beingolea-Guerreiro, 1995; Miranda et al., 1996). In Peru, the migratory grasshoppers Schistocerca piceifrons piceifrons and S. interrita constitute serious agricultural problems since at least 1901 (Beingolea-Guerreiro, 1995). The entomopathogenic fungus Metarhizium anisopliae var. acridum has been found in Peru infecting S. interrita. This pathogen also occurred in northeastern Brazil infecting S. pallens (Moreira et al., 1996) and has been developed as mycopesticide against grasshoppers in Brazil (Magalhães et al., 2000, 2001), Africa (Lomer et al., 1997) and Australia (Milner, 1997). A correct identification and characterization of a particular isolate is very important when developing a pathogen as biocontrol agent.

The objective of this study was to characterize a Peruvian isolate of M. anisopliae var. acridum, obtained from S. interrita, a serious crop pest in Peru.

The fungal isolates used are deposited in the Collection of Entomopathogenic Fungi at Embrapa-Centro Nacional de Pesquisa de Recursos Genéticos e Biotecnologia, Brasília, DF, Brazil (Tigano et al., 2002). The Peruvian isolate of M. anisopliae var. acridum, codified as CG 863, was collected in Peru (Cajamarca State). The identification and characterization were carried out by comparing this isolate with two other ones of M. anisopliae var. acridum, from Brazil (CG 423) and Australia (FI 985), and with an isolate of M. anisopliae var. anisopliae (FI 1029), obtained from another Orthoptera. Cultures were maintained on Sabouraud dextrose agar with 1% yeast extract (SDAY), with a 12-hour photophase at 25ºC.

The conidial morphology following growth on SDAY plates was studied using fresh preparations in water with the aid of a microscope. For each isolate, 50 conidia were measured. The rate of growth on SDAY was assessed by inoculating the center of a Petri plate (90 mm diameter) with a filter paper circle (6 mm diameter) embedded in conidial suspension, and by maintaining the plates at two temperatures, 25°C and 37ºC. For each temperature and isolate, four plates (replicates) were prepared. The radial growth was measured at seven and 14 days after inoculation.

Random amplified polymorphic DNA (RAPD) analysis was conducted according to Welsh & McClelland (1990) and Williams et al. (1990). Mycelium was obtained from a submerged culture in Sabouraud dextrose broth shaked for three days at 150 rpm, at 25°C. Mycelium was harvested by filtration through filter paper (Whatman No. 1), lyophylized and stored at -80°C. Genomic DNA was extracted using a universal rapid salt method (Aljanabi & Martinez, 1997). The PCR reactions were performed in 30-mL volume, with 15 ng of each template, using the PTC-100 programmable thermal controller, and a temperature profile already described (Tigano-Milani et al., 1995). The amplifications were done using the following reaction mix: 1 unit of Taq polymerase, 1x Taq polymerase reaction buffer, 4 mM of MgCl2, 200 mM of each deoxynucleotides triphosphate, and 1 mM of 10-mer primer. Primers were those used by Magalhães et al. (1997) from random primer kit (OPD-92, OPD-011 and OPD-16), and were subjected to electrophoresis in 2% agarose gel dissolved in 0.5x Tris-borato-EDTA (TBE) buffer. After electrophoresis, gels were stained with ethidium bromide (Sambrook et al., 1989) and photographed under UV light. DNA fingerprints were analyzed visually from the photographs.

The insects used in this study were third and fourth instar nymphs of R. schistocercoides, collected in Mato Grosso state (Brazil), reared in cages (50x50x70 cm), maintained at 27ºC in laboratory, and fed with sugarcane leaves, wheat germ, and textured soybean grains. The virulence of the Peruvian isolate against insects, chosen randomly, of R. schistocercoides was compared to the virulence of the Brazilian isolate by two standardized laboratory bioassays according to Magalhães et al. (1997). Briefly, conidia were harvested from the SDAY culture plate with the aid of a brush and suspended in emulsifiable soybean oil. Suspensions were adjusted to 107 conidia/mL and applied topically (3,000 conidia/insect) on the right pleural region of the grasshoppers. After inoculation, insects were holded in cages (17x21x25 cm; 10 insects/cage) and fed as described above. Cages were maintained in the laboratory at 25ºC to 27ºC. Dead insects were recorded each 24 hours and the assays were ended 11 days after inoculation. There were 30 insects per treatment in both bioassays.

The micrometric measurements showed that conidia of the Peruvian isolate (CG 863) are very similar to the conidia of the Brazilian isolate (CG 423) of M. anisopliae var. acridum. They produce ovoid conidia measuring approximately 5x2.7 mm. However, the conidia of the Australian isolate (FI 985) are almost cylindrical, and significantly larger (P<0.01) than the Brazilian and Peruvian isolates (Table 1). These measurements regarding the three isolates are in agreement with the description given by Driver et al. (2000) for the variety acridum of M. anisopliae.

 

 

The growth of the three M. anisopliae var. acridum isolates was alike in agar plates at 25ºC and 37ºC (Table 2). In contrast, M. anisopliae var. anisopliae (FI 1029) had significantly higher rate of growth (P<0.01) than the M. anisopliae var. acridum isolates at 25°C, and did not grow at 37°C. This unusual ability showed by M. anisopliae var. acridum to grow at 37°C had already been reported (Driver et al., 2000).

 

 

The RAPD analysis indicated that the isolates of M. anisopliae var. acridum have similar patterns which are very distinct from the patterns of M. anisopliae var. anisopliae (Figure 1). This observation confirms the usefulness of the RAPD technique to analyze polymorphism amongst Metarhizium varieties (Magalhães et al., 1997; Driver et al., 2000; Milner et al., 2003).

 

 

The Brazilian isolate is as virulent to nymphs of R. schistocercoides as the Peruvian isolate (Figure 2). In another study, Magalhães et al. (1997) found similar results when the isolate CG 423 was compared to the isolate FI 985. Isolates of M. anisopliae var. acridum are found mostly within the Acrididae and it is difficult to find natural infections. In contrast, natural infections caused by the variety anisopliae are easier to find, but their isolates are not as virulent against grasshoppers as the variety acridum (Driver et al., 2000).

 

 

In conclusion, the Peruvian and Brazilian isolates of M. anisopliae var. acridum are very close in terms of growth, conidial morphology and size, RAPD patterns, and activity against R. schistocercoides. This homogeneity among the M. anisopliae var. acridum isolates has been reported in other studies (Magalhães et al., 1997; Drive et al., 2000; Milner et al., 2003). However, further studies including biology, characterization, and activity of a higher number of isolates from other South America regions are needed. Should these traits be confirmed, it would be reasonable to consider the development of a mycopesticide based on a single isolate to control grasshopper pests in the South America continent. This is particularly attractive if the need to lower the production costs and increase market size for biological control products is considered. An immediate consequence of using a single isolate for the entire region would be a significant reduction in the cost of the registration process, one of the most expensive steps in developing a mycopesticide.

 

Acknowledgements

To John Capinera, Entomology Department, University of Florida, and Marcos Faria, Embrapa-Centro Nacional de Pesquisa de Recursos Genéticos e Biotecnologia, for reviewing the manuscript; to Ana Cristina Oliveira Teixeira, for technical support; to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), for partial support of the work.

 

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Accepted for publication on September 10, 2003

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